Population= a collection of individuals of the same species occupying the same space at the same time
individual can be derived from sexual reproduction, or from clonal reproduction.  Individuals originating from seeds are usually, by not always the result of sexual reproduction.  Apomixis is the general term for asexual reproduction, but some people use it to refer only to when a seed develops w/o fertilization.

Clonal reproduction can occur from rooted aboveground (stolons) or belowground stems (rhizomes), and bulbs and bulbils
individuals established by sexual reproduction = genets;
individuals established by clonal reproduction = ramests

Benefit/cost of sexual reproduction:
increases genetic diversity and thus phenotypic diversity- important if environmental variation is stochastic (or unpredictable)

resource investment into flower, fruit, and seed is significant

Benefits/cost of clonal reproduction:

 rate of potential growth of new ramets is high
 plant can "forage" for resources in a heterogeneous environment
 clonal plants tend to be better competitors
 mortality of young is lower than for seedlings

Lack of genetic diversity can be detrimental if rare environmental condition occurs (e.g. pathogens, climatic stress)
Dispersal is more constrained

Some characteristics of populations (demography):
A.  Population Density
Number of individuals per unit area; indicative of:
 1) habitat quality- amount of resources and climatic conditions
 2) reproductive potential of the population

B.  Spatial distribution of individuals
 1) regular or uniform spacing
suggests: a) competitive interactions are important or b) allelopathy

2) clumped distribution
suggestive of: 1) low seed dispersal distance, and/or 2) strong microhabitat preference

3) random distribution - spacing of one individual is independent of another individual; very rare occurrence; indicates no biotic interactions

C.  Age/Stage structure
Individuals are often divided according to age class, to better characterize reproductive potential and mortality rates
Aging is difficult in plants, except for some woody species
"Stage" may be a better descriptor= seed, seedling, vegetative, reproductive, senescent… (lots of possible variants)

Population growth models

Change in population size occurs as a result of:
 1) births (germination in plants)
 2) death
 3) immigration- seeds from other populations
 4) emigration - seeds leaving the population

immigration & emigration often ignored, except for estimating genetic structure; many models assume immigration=emigration

let  b = rate of births per individual in the population
 d = rate of deaths per individual
 r, the intrinsic rate of population increase, = b - d
 N = size of the population
 t = time
then:
  d N / d t = r N

if b > d (and thus r is +), then the rate of population growth will be exponential

exponential population growth can not be maintained indefinitely- resources are finite, and at some population size, competition within a population and among populations will occur;  amount of herbivory and frequency of disease is related to the population size
= density dependent factors- i.e. the intensity of controls on population growth are dependent on the size of the population

Density dependence will result in r changing with increasing N; as N increases, r will decrease

Populations will just sustain themselves if r = 1; size is called the "carrying capacity" (K)

Exponential growth equation can be modified by a term that takes
 the carrying capacity concept into account:

  1 - N / K  ; as N goes to K, this term approaches 0

thus:   dN / dt = rN (1-N/K) provides a growth model that gives a density dependent control on r; Logistic equation

most "real" populations show some cycling around "K", due to
 1) varible climate and resource supply
 2) feedbacks with temporal lags-e.g. herbivore populations that have a delay between when plant population increases and increased births occur
 3) differential contribution to recruitment (and mortality) from different age classes

6 Oct 05